The field of this invention involves shape-memory-effect (SME) actuators, and in particular those usages of shape-memory-alloy as they apply to making linear electro-mechanical actuators. Although rotary, torsional and other devices and other configurations are within the scope of the invention, this specification will limit itself to the preferred linear embodiments.
Shape-memory alloys have been used for actuator-type devices previously. Generally, the material is a nickel-titanium alloy called Nitinol or Tinel.RTM.*, although copper-based alloys have been used in many similar applications. The material has been used for actuators in relays according to Jost (U.S. Pat. No. 3,968,380), Hickling (U.S. Pat. No. 3,849,756), Sims (U.K. Application No. 2,026,246), and Clarke (U.S. Pat. No. 3,872,415). It has been used in temperature-sensing actuators as described by Levinn (U.S. Pat. No. 3,371,247), DuRocher (U.S. Pat. Nos. 3,707,694 and 3,676,815), Wilson (U.S. Pat. Nos. 3,652,969, 3,634,803 and 3,594,674U), and Melton (U.S. Pat. No. 4,205,293). An SME valve actuator has been described by Wilson (U.S. Pat. No. 3,613,732). FNT *Tinel is a Registered Trademark of Raychem Corporation.
Clark (U.S. Pat. No. 3,948,688) describes a technique for conditioning and improving the fatigue life of a shape-memory alloy by thermally cycling the material while "the alloy is maintained under a tensile stress sufficient to strain it beyond its plastic yield point" (see Abstract). This technique is described as improving the alloy characteristics before it is designed into a device, whereas the current invention is intended to ensure that the alloy does not exceed its design criteria via some unpredicted force and suffer damage which will limit its useful life to a value shorter than that for which it was intended.
A similar arrangement is taught by Sims (U.K. Application No. 2,026,246) wherein a compression accessory spring biases a shape-memory-alloy spring in tension (see page 2, lines 1-10).
Hickling (U.S. Pat. No. 3,849,756) teaches the use of an accessory spring both for moving SME actuators "back to the undeformed state" (that is, a return or reset spring) (see Col. 9, lines 37-40) and also for a tensioning or bias spring to keep a "structural member . . . in that position" (see Col. 9, lines 14-18).
Levinn (U.S. Pat. No. 3,731,247) uses accessory springs both as a return or reset spring as previously described and also as a means for limiting the movement of a wire of shape-memory alloy. In this case, a straight wire is heated over only a part of its length. The movement or recovery upon heating over that fraction of the total length is sufficient to actuate a switch. The wire may, however, be heated over a longer length (as anticipated by the design) than required to just throw the switch. The accessory spring in series with the wire is used to limit the movement of the wire to only that amount necessary to throw the switch. In so doing, it assures that "no damage will be done to the system". The instant invention differs in several respects from this. First, the instant invention attempts to protect an actuator against unexpected, not anticipated, events that could cause damage. Second, because the current invention is connected, in the usual embodiment, to another mechanism, it similarly protects againt damage to the outside mechanism as well as damage to itself. Third, the use of an accessory spring in series with the shape-memory-alloy spring could make the device sufficiently long as to make it impractical. The instant invention utilizes a coaxial embodiment which minimizes the length of the device and therefore conserves space. Fourth, the alloy spring of the current invention is designed to recover completely, not partially.